Control systems for industrial processes involve the monitoring of process variables and the actuation of process control devices to alter or maintain such process variables. Process variables include temperature, flow, mixture, etc. which can be affected by a drive unit which regulates, for example, a heater, a valve, or the addition of a component. Such process controls are usually distributed throughout a plant or along an assembly line. Therefore, individual stations for monitoring and/or actuation of process control devices are employed. In order to assure that processes are properly controlled, the individual stations must be able to communicate with each other and are frequently connected in a network. The loss of communication between stations or the loss of individual stations can adversely affect the process being controlled, resulting in loss of control of the process or damage to the plant. Because these processes are frequently dynamic and involve rapid or instantaneous changes, it is important that information be passed between stations in real time. Equally important, station or connector failures should be detected rapidly and corrective action taken, such as process shutdown or maintenance of the status quo. Maintaining of the network connection by self-healing is a desirable feature.
Similarly, communications systems involve the sending and receiving of signals between stations, frequently in real time. It is also desirable that a communication network fault be detected and corrective action, such as self-healing, initiated.
The sending and receiving of process control or communication signals often occurs in an environment containing electromagnetic fields which can interfere with the transmission of signals. A typical process control or communication system would involve the use of copper wire connectors for process controllers which are arranged in a bus, a ring or a star configuration. Frequently, there are one or more master stations, which control the remaining slave stations. In such an arrangement, failure of a master station can result in failure of the entire network. Alternatively, networks comprised of peer stations can be constructed. However, all peer networks involve problems associated with synchronization of the data signals and the control of the network to avoid data conflicts. To deal with station or connector failures, a typical solution is to send and receive failure signals and acknowledgment signals, with networks having slave stations dependent on the master stations for configurational control or self-healing of the network.
Many existing systems require synchronization of transmissions between the individual stations, typically implemented by phase-lock loops. Some form of monitoring of data on the network is required in order to avoid conflicts with data being introduced to or removed from the network.